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Equity-settled share-based payments - Restricted Share and Restricted Stock Unit Plan (Details)
12 Months Ended
May 15, 2017
USD ($)
Apr. 01, 2017
USD ($)
May 15, 2016
USD ($)
Apr. 01, 2016
USD ($)
May 15, 2015
USD ($)
Apr. 01, 2015
USD ($)
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Share-based compensation expense | $             $ 5,552,000 $ 4,796,000 $ 4,396,000
Grant date, fair value (usd per share) | $ $ 12.14 $ 11.88 $ 12.52 $ 12.63 $ 8.62 $ 9.45      
Possibility of ceasing employment before vesting 0.00% 0.00% 0.00% 5.00% 0.00% 5.00%      
Restricted Stock And Restricted Stock Units (RSU) [Member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Vesting period (in years)             3 years    
Vesting percentage             33.00%    
Maximum shares authorised             2,474,701    
Restricted stock units                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Share-based compensation expense | $             $ 5,600,000 $ 4,800,000  
Number of shares outstanding, Jan 1             1,000,000 1,018,000 861,000
Number of shares granted             488,000 464,000 626,000
Number of shares forfeited             (29,000) (29,000) (37,000)
Number of shares vested             (490,000) (453,000) (432,000)
Number of shares outstanding, Dec 31             969,000 1,000,000 1,018,000